Terminal units of the prior art utilize an analog form of autostart circuit that is known as mark autostart. Terminal units convert RTTY mark and space tones transmitted via radio to digital pulse which are useful for driving an output device such as the teleprinter. Output devices such as teleprinters and cathode ray terminals respond to information which is digitally coded usually in a commonly known code such as the Murray or ASCII code. Each letter and figure and other character has a start and a stop pulse with a unique combination of binary coded pulses between the start and stop pulses. In the Murray code five bits are used to uniquely define each letter and figure and in the ASCII, seven bits, the eighth being a parity bit. In transmitting these coded signals via radio the highs and lows of the binary bits of each character are converted to mark and space tones which differ by a predetermined frequency. The terminal unit conventionally has a separate mark and space channel which reconverts tones of a particular frequency back into pulses. An autostart circuit is a circuit for automatically energizing an output device so long as an input signal to the circuit is present.
In the prior art the energy signals in the mark channel of the terminal unit rather than the space channel are utilized as an autostart signal according to the convention commonly in use in transmitting radio teleprinter codes. When no information is being transmitted, it is conventional to transmit a mark rather than a space signal. Mark autostart is characterized by its response to any amplitude energy received in the filter of the mark channel of the terminal unit. Because it responds to any amplitude energy, it is susceptible to false starts caused by single sideband signals, CW (Morse Code) signals, noise crashes, static and radio frequency carrier signals which are varied in frequency through the frequency in which the mark signal is received. A mark autostart circuit does not respond to energy in the space channel of a terminal unit. Nor is there any detection of information characters encoded in a radio teleprinter code to differentiate such characters from other signals received at any speed or substantially at a predetermined speed.
In the prior art there are circuits which respond to particular combinations of letters and figures which are known as Selcal circuits for selective calling a particular radio station identified by a unique call sign. Such Selcal circuits are complicated by the lack of exact logic needed to implement them and have no general application for the purpose of energizing an output device whenever an incoming signal is present whether or not an individual station is called.